Charlie Bird, I wouldn't worry quite yet. I would consider buying now to be safe in terms of having enough time to receive the PE and upgrade it.
If you're concerned, you can always contact Entourage directly and say you're this close to pressing "Buy" but need to know that you'll have access to the Ermine beta. You probably also will have enough of a return period with your device to be able to send it back if by some chance (which I honestly think is quite small) you get the device and then can't receive the update from Entourage.
My recommendation, if you've decided you already want it and are just worried about this one issue alone, is to buy. But that's just me.